Broadband networks are systems of network components which receive and/or transmit broadband signals where the signals are analog wave forms within the 5 MHz (megahertz) to 1 GHz (gigahertz) frequency range; contain information encoded with analog modulation; and are combined through multiplexing (typically, frequency division multiplexing). These network components are interconnected through network connections. Examples of broadband networks include CATV/MATV (Community Access Television, Multiple Access Television) systems and data networks. A CATV/MATV system is typically composed of one or more "head-ends" which deliver television channels to a community of homes over an HFC (hybrid-fiber coax) infrastructure. The network components in a CATV/MATV broadband network include RF (radio-frequency) modulators, RF demodulators, frequency converters, band-pass filters, band-trap filters, combiners, splitters, taps, attenuators, equalizers, amplifiers, broadband switches, fiber-optic nodes, and metering equipment. These components are connected to each other through transmission lines which are typically coaxial cable.
A broadband network will typically be spread over a large physical area, passing hundreds of thousands of residential homes and commercial businesses. Due to the breadth of the network and the complex nature of the environments the broadband network encompasses, interference of signals on the broadband network is of a large concern to the engineers maintaining the broadband network. Noise may enter into a broadband network in many ways. For instance, if a cable on the broadband network is not properly terminated, that cable can act as an antenna and allow outside signals to enter into the broadband network. The cable may also cause portions of the broadband signal to be reflected back into the network at the termination due to the impedance mismatch of an improper termination. The long lengths of cable run to each home can also act as antennas and allow outside broadband signals, such as those from CB radios, to enter into the network. Further, the cables and equipment themselves introduce attenuation and noise into the system. An engineer at a cable system quickly learns to identify and compensate for the predictable sources of noise within the broadband network. However there is a type of noise, called transient noise (sometimes also referred to as impulse noise), which is unpredictable, short-lived, and difficult to work around.
Transients are short lived changes of voltage, frequency and/or amplitude which interfere with broadband signals. Transients may be generated from neon signs and/or vehicle ignitions. Transients may also be generated from devices such as televisions, appliances, lighting equipment, and cable modems being turned on and off or from devices which have poor wiring (e.g. loose connections). The duration of a transient is often related to the size of the electronic device causing the transient. i.e., a large capacitor will often cause a lengthy (approximately 100 milliseconds) transient, a small capacitor may be the cause of a short (approximately 1 millisecond) transient. As electronic devices are reduced in size, it is expected that the duration of transients will also be reduced.
The head-end of a broadband network typically transmits television channels and other outbound data within the broadband spectrum of 54-750 MHz. This forward spectrum is amplified and split through a tree-and-branch configuration to be presented to each customer (residence, business) of the broadband network. Increasingly, broadband networks are being configured to allow certain customers to transmit television channels and other data back through the tree-and-branch network to the head-end. These customers transmit within a return spectrum of 5-50 MHz. Typical information transmitted within the return spectrum is locally generated television channels, requests from converter boxes to view pay-per-view events, and computer information requests from cable modems such as requests for internet access or requests for world wide web pages. Examples of customers are schools which transmit television broadcasts of sporting events; town halls which transmit broadcasts of town meetings; and residential homes which are equipped with cable modems that transmit (and receive) digital computer data. Unfortunately the 5-50 MHz range of the broadband spectrum is very susceptible to transient noise. And because of the tree-and-branch architecture of a typical broadband network, the noise (transient or otherwise) present on one branch of the network may get accumulated with noise present on other branches of the network during its transmission from the sources to the head-end. This accumulated noise may affects the broadband signals transmitted on all the combined branches. Hence, noise present on one branch of a broadband network can interfere with signals present on a sibling branch.
A head-end will typically have a device such as a television demodulator or a cable modem which listens for signals within the return spectrum of the broadband network to receive return transmissions generated from the customers. A head-end may, for example, receive a sports event broadcast from a local school within the spectrum range of 30-36 MHz and rebroadcast that television broadcast onto a forward channel to its subscriber base. If transient noise interferes with the 30-36 MHz signals, that noise may interfere with the broadcast picture or sound by introducing sparkles or pops and a degraded signal will be broadcast to the cable systems customers. Of even more concern to a cable system, is the effect of transient noise on portions of the return spectrum containing digital data. Cable systems may be equipped with cable modems which receive digital information transmitted by the subscribers. This digital information can be information from a cable pay-per-view converter box, a set-top-box, or a personal computer. When digital information transmitted by a subscriber is corrupted due to noise, that information is lost and must be retransmitted. Continual retransmission of digital data within the return spectrum of a broadband network cuts down on the effective bandwidth of the network. If transient noise can be minimized, the return spectrum of the broadband network can be used more effectively and efficiently.
Several metering devices are capable of monitoring a broadband network for transient signals. These devices typically are composed of a carrier generator and a comparator. The carrier generator is placed in the vicinity of a suspected noisy branch of the broadband network and transmits a high quality test signal. The comparator is located upstream of the noisy branch, typically at the head-end of the network, and monitors the received test signal, looking for perturbations. Through careful selection of carrier generators and comparators, the presence of many types of transient noise can be identified Types of comparators include phase and amplitude difference detectors (such as the CW Tester developed by CableLabs) and power detectors. Cable modems themselves can also be used as comparators when they are configured to report on the number of digital packets lost.
Because transient signals are of a very short duration, special equipment is needed to analyze and characterize them. An engineer of a broadband network will typically use a spectrum analyzer to diagnose and repair problems on the network. However, because the spectrum analyzer displays the average amount of energy on a given frequency over time, these analyzers are not effective tools for trouble shooting transient noise problems. A transient fluctuation of a signal may have come and gone during the time a spectrum analyzer samples the energy of the perturbed frequency. The normal, non-perturbed, signal may preside over the perturbed signal and yield an average signal strength which is within acceptable limits. Specialized devices such as digital oscilloscopes which can monitor in great detail changes in energy over a limited portion of frequency bandwidth are needed to characterize transient noise problems on broadband networks.
See the Applications and Technology article "Delivering Two-Way Service" published by Hewlett Packard for a discussion of the noise ingress found on cable systems. And, see the three part series, "Insights into proper return path alignment", "Proactive return path maintenance", and "Noise and ingress performance in the return path" also published by Hewlett Packard for a discussion of common techniques for measuring, monitoring, and analyzing noise present in a broadband network. There references are herein incorporated by reference in their entirety.